Russia began delivery of an advanced missile defence system to Turkey on Friday, a move expected to trigger U.S. sanctions against a NATO ally and drive a wedge into the heart of the Western military alliance.

The first parts of the S-400 air defence system were flown to a military air base near the capital Ankara, the Turkish Defence Ministry said, sealing Turkey’s deal with Russia that Washington had struggled for months to prevent.

The United States says the Russian military hardware is not compatible with NATO systems and the acquisition may lead to Ankara’s expulsion from an F-35 fighter jet program.

Investors in Turkey have been unsettled by the deal. The Turkish lira weakened against the U.S. dollar before the ministry announced the arrival of the S-400 consignment to the Murted Air Base, northwest of Ankara.

“The delivery of parts belonging to the system will continue in the coming days,” Turkey’s Defence Industry Directorate said. “Once the system is completely ready, it will begin to be used in a way determined by the relevant authorities.”

At least two Russian Air Force AN-124 cargo planes flew to Turkey on Friday morning, data from plane tracking website Flightradar24 showed. Turkish broadcasters showed footage of one plane parked at airbase and a second one landing at around 12.30 p.m. local time.

Russia’s Federal Service for Military-Technical Cooperation confirmed on Friday it had started delivering the S-400 systems and the deliveries would continue in accordance with an agreed schedule, the RIA news agency reported.

Turkey says the system is a strategic defence requirement, particularly to secure its southern borders with Syria and Iraq. It says that when it made the deal with Russia for the S-400s, the United States and Europe had not presented a viable alternative.

Fighter jet deal could be off

President Tayyip Erdogan said after meeting President Donald Trump at a G20 summit last month that the United States did not plan to impose sanctions on Ankara for buying the S-400s.

Trump said Turkey had not been treated fairly, but did not rule out sanctions. U.S. officials said last week the administration still plans to impose sanctions on Turkey.

Russian President Vladimir Putin, right, and Turkish President Recep Tayyip Erdogan at the G20 summit in Osaka, Japan last month. The countries have come to agreements on military equipment and monitoring and enforcing ceasefires in Syria. (Yuri Kadobnov/Pool via Reuters)

Under legislation known as Countering America’s Adversaries through Sanctions Act (CAATSA), which targets purchases of military equipment from Russia, Trump should select five of 12 possible measures.

These range from banning visas and denying access to the U.S.-based Export-Import Bank, to the harsher options of blocking transactions with the U.S. financial system and denying export licences.

In the first comments from Washington, acting Defence Secretary Mark Esper said the U.S. position on the issue has notchanged.

“We are aware of Turkey taking delivery of the S-400, our position regarding the F-35 has not changed, and I will speak with my Turkish counterpart Minister Akar this afternoon,” Esper said. “There will be more to follow after that conversation.”

Washington has said the S-400s could compromise its Lockheed Martin F-35 stealth fighter jets, an aircraft Turkey is helping to build and planning to buy.

Turkey could also face expulsion from the F-35 program under the sanctions. Erdogan has dismissed that possibility, but Washington has already started the process of removing Turkey from the program, halting training of Turkish pilots in the United States on the aircraft.

Investors in Turkey have been concerned about the impact of potential U.S. sanctions on an economy that fell into recession after a currency crisis last year.

Also: Syria, Gulen oil

The S-400 acquisition is one of several issues that have frayed ties between the two allies, and has worried some in the West that Turkey is drifting closer to Moscow’s sphere of influence.

That includes a dispute over strategy in Syria east of the Euphrates River, where the United States is allied with Kurdish forces that Turkey views as terrorists. In Syria, Turkey supports the opposition against President Bashar al-Assad, but has joined with Russia to secure and monitor local ceasefires.

Turkey has also long demanded Washington hand over a Muslim cleric, Fethullah Gulen, which Ankara holds responsible for an attempted coup in 2016.

U.S. officials have said the courts would require sufficient evidence to extradite the elderly Gulen, who resides in Pennsylvania and has denied any involvement in the coup.

The United States reimposed sanctions on Iran last year, barring countries from importing its oil. In May Washington scrapped a six-month waiver granted to Turkey and seven other big importers in order to step up attempts to isolate Tehran and choke off its oil revenues.

Turkey, which complained but fully complied with the sanctions, is dependent on imports for almost all of its energy needs and Iran is a leading gas and oil supplier.